Add wxTextOutputStream::Write<>().

The new method allows to write into wxTextOutputStream anything that can be
streamed into wxString and allows to simplify the existing code.

interface/wx/txtstrm.h

@@ -300,6 +300,14 @@ public:
     */
     void SetMode(wxEOL mode = wxEOL_NATIVE);
 
+    /**
+        Writes @a i to the stream using wxString::operator<<().
+
+        @since 3.1.0
+    */
+    template<typename T>
+    void Write(const T& i);
+
     /**
         Writes the 64 bit integer @a i64 to the stream.